1. Field of the Invention
The present invention relates to an optical information recording medium, and in particular to improvements in heat-mode writable optical information recording media.
2. Description of the Related Art
A CD-R is a widely known writable optical information recoding medium (optical disc) on which information can be written only once by irradiation with laser light. A CD-R typically has a recording layer containing an organic colorant, a light-reflective layer containing metals such as gold, and a protective layer composed of resins, laminated in this order on a transparent disc substrate. Information is recorded on the CD-R by irradiating it with near-IR laser light (generally having a wavelength of around 780 nm). Specifically, the recording layer of the optical disc absorbs light which causes a temperature increase at an exposed portion, resulting in a physical or chemical change (for example, formation of pits) and, in turn, a change in the optical properties of that portion, whereby information data are recorded at that portion. On the other hand, when reading data from the thus-recorded optical disc, the CD-R is, in general, irradiated with laser light having the same wavelength as that of the light used for information recording, and a difference in reflectance is detected between at the portions having (recorded area) and those not having (non-recorded area) the optical change produced in the recording layer.
Optical information recording media with higher density recording capabilities are desired these days. To meet this requirement, an optical disc of a different type, a writable digital versatile disc (DVD-R) has been proposed (for example, in a separate volume of Nikkei New Media, xe2x80x9cDVDxe2x80x9d, published in 1995). A DVD-R is so constructed that two discs each having an organic colorant-containing recording layer, a light-reflective layer and a protective layer are laminated in this order on a transparent disc substrate are laminated with the recording layers of the two discs facing inside, or is so constructed that one recording disc of the type described above is laminated with a protective disc substrate having the same form as that of the recording disc, also with the recording layer facing inside. In the DVD-R, the transparent disc substrate is processed so as to have a narrow guide groove (pre-groove) for laser light tracking, and a width of the groove, which is, for example, from 0.74 to 0.8 xcexcm is less than half as wide as that in a CD-R. Information recording and reading are performed by irradiating the DVD-R with visible laser light (generally having a wavelength within a range of from 630 nm to 680 nm), and higher density recording is possible than on a CD-R.
Recently, networks such as the internet and high-definition television (HDTV) are rapidly being popularized, and full-scale telecasting for HDTV is not far away. Given this situation, large-scale recording media capable of having image information recorded thereon at low cost and in a simple manner are needed. At present, the DVD-R may perform its role as a large-capacity recording medium, but demand for further increasing the recording capacity and density of recording media is now increasing more and more, and it is necessary to develop recording media that meet this demand. For this reason, a recording medium of higher capacity and capable of undergoing higher density recording by irradiation with light having a shorter wavelength than that used with DVD-Rs is now being developed.
For example, Japanese Patent Application Laid Open (JP-A) Nos. 4-74690, 7-304256, 7-304257, 8-127174, 11-53758, 11-334204, 11-334205, 11-334206, 11-334207, 2000-43423, 2000-108513, 2000-113504, 2000-149320, 2000-158818 and 2000-228028 disclose methods for recording and reading information by irradiating optical information recording media, having an organic colorant-containing recording layer, with laser light, having a wavelength no longer than 530 nm, from a recording layer side thereof toward the light-reflective layer side thereof. In these methods, an optical disc having a recording layer that contains any of porphyrin compounds, azo dyes, metal azo dyes, quinophthalone dyes, trimethinecyanine dyes, dicyanovinylphenyl skeleton-having dyes, coumarin compounds or naphthalocyanine compounds is irradiated with blue (430 nm, 488 nm) or blue-green (515 nm) laser light for recording information thereon and reading information therefrom.
From a viewpoint of compatibility with the current CD-R system, optical information recoding media capable of recording and reading via laser light of different two wavelength ranges are proposed. For example, JP-A Nos. 2000-141900, 2000-158816, 2000-185471, 2000-289342 and 2000-309165 propose optical information recording media capable of recording and reading with any laser light of a near-IR laser light range of around 780 nm or visible laser light range of around 650 nm, by mixing the colorants used in CD-Rs and those used in DVD-Rs.
However, the present inventors have found through investigation that sensitivity of the optical discs described in the above-mentioned patent publications is still not of a practical level when information is recorded thereon via irradiation with short-wave laser light of a wavelength no longer than 450 nm. In addition other recording characteristics such as reflectivity and modulation of the optical discs are not satisfactory, and therefore the optical discs must be further improved. In particular, present inventors have confirmed that recording characteristics of the optical discs described in the above-mentioned patent publications are poor when they are irradiated with laser light of no longer than 450 nm.
On a surface opposite to the substrate of the optical discs mentioned above, a transparent cover layer is generally formed. Since the cover layer absorbs short-wave or long-wave light, the colorants, such as phthalocyanine dyes, contained in the recording layer of the optical discs are decomposed over time by light that passes through the cover layer, and the storage stability of the optical discs is often inferior.
On the other hand, in fabricating a DVD, a tabular substrate having a thickness of, for example, 0.6 mm is adhered to a dummy substrate having a thickness of, for example, 0.6 mm, and necessary visible information and decoration for increasing commercial value are printed on the surface of the dummy substrate. This printing on the dummy substrate is carried out via various printing methods such as pad printing, screen printing or offset printing for full-color prints, and the same printing pattern is used for all recording discs produced on one production line.
As mentioned above, disc recording media are generally formed of a transparent polycarbonate resin. Therefore, if surfaces thereof do not have decoration or titles printed thereon, one can not be differentiated from another, and in addition, they leave an uninteresting impression and therefore Often have low commercial value. However, if the printing pattern is varied for every recording disc when a conventional pad printing or screen printing method is used, different printing plates must be prepared for all of the intended printing patterns, and this is costly and time consuming. Therefore, it is impossible to readily change the contents of the printing, and for prints of many colors, increased cost and complexity of the printing process represent another problem.
For these reasons, it is realistically difficult to print different patterns on different recording discs of the same type so as to individually differentiate them by the printed patterns, and thus the same pattern is printed on all of the recording discs of the same type.
One object of the present invention is to provide an optical information recoding medium having stable recording and reading characteristics and having good storage stability, including light resistance and heat resistance, by preventing colorants in a recording layer therein from being decomposed.
Another object of the invention is to make it possible to readily fabricate recording discs of different appearances that can be individually differentiated from each other and to thereby further increase the commercial value of the recording discs.
The above-stated objects are attained by the invention described below. Specifically, in a first aspect thereof, the invention provides an optical information recording medium having, on a substrate, at least a light-reflective layer, a recording layer on which information can be recorded via laser light of wavelength no longer than 500 nm, and a cover layer containing at least one of UV absorbents that block out light of wavelength no longer than 400 nm and dyes that block out light of wavelength no shorter than 500 nm, formed in this order.
In a second aspect thereof, the invention provides a disc recording medium having a recording layer on which information can be recorded and reading the recorded information can be reproduced via irradiation with light, wherein a substrate opposite to a disc surface that is irradiated with light contains a colorant material.
This makes it possible to readily fabricate recording discs of different appearances and to differentiate the individual discs from each other. In addition, the substrate opposite to the disc surface that is irradiated with light is colored. Printing of background color on the discs can therefore be omitted, and a printing step only requires printing necessary information thereon. Printing frequency can be reduced or printing can be completely omitted, thus simplifying the process of fabricating the recording discs.
If desired, the substrate opposite to the disc surface that is irradiated with light may contain two or more different colorant materials, and different patterns can be formed on different substrates. Accordingly, each optical disc recording medium may have a multi-color marble pattern, enhancing its appearance, and increasing its commercial value